U.S. patent application number 14/629369 was filed with the patent office on 2015-08-27 for electrophoretic display.
The applicant listed for this patent is E Ink California, LLC. Invention is credited to Hui Du, Peter Laxton, Yu Li.
Application Number | 20150241754 14/629369 |
Document ID | / |
Family ID | 53879102 |
Filed Date | 2015-08-27 |
United States Patent
Application |
20150241754 |
Kind Code |
A1 |
Du; Hui ; et al. |
August 27, 2015 |
ELECTROPHORETIC DISPLAY
Abstract
The present invention is directed to an electrophoretic display
device comprising an electrophoretic fluid which fluid comprises
two type of charged particles dispersed in a solvent, wherein the
two types of charged particles are of the same color, but carrying
opposite charge polarities.
Inventors: |
Du; Hui; (Milpitas, CA)
; Laxton; Peter; (Alameda, CA) ; Li; Yu;
(Fremont, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
E Ink California, LLC |
Fremont |
CA |
US |
|
|
Family ID: |
53879102 |
Appl. No.: |
14/629369 |
Filed: |
February 23, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61943926 |
Feb 24, 2014 |
|
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Current U.S.
Class: |
359/296 |
Current CPC
Class: |
G02F 1/167 20130101;
G02F 2001/1678 20130101; G02F 1/1681 20190101; G02F 1/1676
20190101 |
International
Class: |
G02F 1/167 20060101
G02F001/167 |
Claims
1. An electrophoretic display comprising an electrophoretic fluid
which fluid comprises two type of charged particles dispersed in a
solvent, wherein a) the two types of charged particles are of the
same color, but carrying opposite charge polarities; and b) the
color difference between the on and off states of a pixel,
expressed in .DELTA.E, is at least 5.
2. The display of claim 1, wherein .DELTA.E is at least 10.
3. The display of claim 1, wherein the solvent has a color which is
visually distinguishable from the color of the charged
particles.
4. The display of claim 1, wherein the fluid further comprises
uncharged or slightly charged particles which have a color visually
distinguishable from the color of the charged particles.
5. The display of claim 4, wherein the solvent is clear and
colorless.
6. The display of claim 3, wherein the fluid further comprises a
charge control agent.
7. The display of claim 4, wherein the fluid further comprises a
charge control agent.
8. The display of claim 3, wherein the charged particles are of a
white color and the solvent is dyed red, green or blue.
9. The display of claim 4, wherein the charged particles are of a
white color and the uncharged or slightly charged particles are
red, green or blue.
10. The display of claim 1, wherein the charged particles are
coated with a solvent-soluble polymer.
11. The display of claim 1, wherein the fluid is sandwiched between
a common electrode and a plurality of pixel electrodes and the
common electrode has a dielectric coating.
12. The display of claim 1, wherein the density of the charged
particles is at least twice as high as the density of the solvent
in which the particles are dispersed.
13. The display of claim 1, wherein the fluid has a low shear
viscosity in the range of 0.5 to 50 cps, at room temperature.
14. The display of claim 1, wherein the size of the charged
particles is larger than 1 micron.
15. The display of claim 1, wherein the fluid is filled in display
cells.
16. The display of claim 15, wherein the display cells are cup-like
microcells.
17. The display of claim 15, wherein the display cells are
microcapsules.
Description
[0001] The present application claims the benefit of U.S.
Provisional Application No. 61/943,926, filed Feb. 24, 2014, the
content of which is incorporated herein by reference in its
entirety.
FIELD OF THE INVENTION
[0002] The present invention is directed to an electrophoretic
display device and an electrophoretic fluid for such a display
device.
BACKGROUND OF THE INVENTION
[0003] An electrophoretic display (EPD) is a non-emissive device
based on the electrophoresis phenomenon influencing charged pigment
particles dispersed in a dielectric solvent. An EPD typically
comprises a pair of spaced-apart plate-like electrodes. At least
one of the electrode plates, typically on the viewing side, is
transparent. An electrophoretic fluid composed of a dielectric
solvent with charged pigment particles dispersed therein is
enclosed between the two electrode plates.
[0004] An electrophoretic fluid may have one type of charged
pigment particles dispersed in a solvent or solvent mixture of a
contrasting color. In this case, when a voltage difference is
imposed between the two electrode plates, the pigment particles
migrate by attraction to the plate of polarity opposite that of the
pigment particles. Thus, the color showing at the transparent plate
may be either the color of the solvent or the color of the pigment
particles. Reversal of plate polarity will cause the particles to
migrate back to the opposite plate, thereby reversing the
color.
[0005] Alternatively, an electrophoretic fluid may have two types
of pigment particles of contrasting colors and carrying opposite
charges, and the two types of pigment particles may be dispersed in
a clear solvent or solvent mixture. In this case, when a voltage
difference is imposed between the two electrode plates, the two
types of pigment particles would move to the opposite ends. Thus
one of the colors of the two types of the pigment particles would
be seen at the viewing side.
[0006] An electrophoretic display typically exhibits bistability,
which means that after charged particles in an electrophoretic
fluid are driven to desired locations under an electric field, the
charged particles would remain substantially unmoved after the
electric field is turned off. In other words, when an image is
displayed by driving charged particles to the intended locations,
the image would remain unchanged even after the electric field is
turned off.
BRIEF DISCUSSION OF THE DRAWINGS
[0007] FIG. 1 illustrates an electrophoretic fluid of the present
invention.
[0008] FIG. 2 illustrates how different color states may be
displayed by a display device of FIG. 1.
[0009] FIG. 3 illustrates an alternative electrophoretic fluid.
[0010] FIG. 4 illustrates how different color states may be
displayed by a display device of FIG. 3.
DETAILED DESCRIPTION OF THE INVENTION
[0011] A first aspect of the present invention is directed to an
electrophoretic fluid, as shown in FIG. 1.
[0012] In FIG. 1, a display device utilizes an electrophoretic
fluid which comprises two types (11 and 12) of charged particles of
the same color, but carrying opposite charge polarities. The two
types of charged particles are dispersed in a solvent or solvent
mixture (13) which has a color visually in contrast with the color
of the two types of charged particles.
[0013] The display fluid is sandwiched between two electrode
layers. One of the electrode layers is a common electrode (15)
which is a transparent electrode layer (e.g., ITO), spreading over
the entire top of the display device. The other electrode layer
(16) is a layer of pixel electrodes (16a). The space between two
dotted vertical lines represents a pixel. Therefore each pixel has
a corresponding pixel electrode.
[0014] The pixel electrodes (16a) are described in U.S. Pat. No.
7,046,228, the content of which is incorporated herein by reference
in its entirety. It is noted that while active matrix driving with
a thin film transistor (TFT) backplane is mentioned for the layer
of pixel electrodes, the scope of the present invention encompasses
other types of electrode addressing as long as the electrodes serve
the desired functions.
[0015] The concentration of the charged particles may be 1% to 20%
by volume, preferably 5% to 10% by volume, in the display
fluid.
[0016] If the charged particles are of a white color, they may be
formed from an inorganic pigment such as TiO.sub.2, ZrO.sub.2, ZnO,
Al.sub.2O.sub.3, Sb.sub.2O.sub.3, BaSO.sub.4, PbSO.sub.4 or the
like. They may also be polymer particles with a high refractive
index (>1.5) and of a certain size (>100 nm) to exhibit a
white color.
[0017] For the black charged particles, they may be formed from CI
pigment black 26 or 28 or the like (e.g., manganese ferrite black
spinel or copper chromite black spinel) or carbon black.
[0018] If the charged particles are of other colors (non-white and
non-black), they may be formed from organic pigments such as CI
pigment PR 254, PR122, PR149, PG36, PG58, PG7, PB28, PB15:3, PY83,
PY138, PY150, PY155 or PY20. Those are commonly used organic
pigments described in color index handbooks, "New Pigment
Application Technology" (CMC Publishing Co, Ltd, 1986) and
"Printing Ink Technology" (CMC Publishing Co, Ltd, 1984). Specific
examples include Clariant Hostaperm Red D3G 70-EDS, Hostaperm Pink
E-EDS, PV fast red D3G, Hostaperm red D3G 70, Hostaperm Blue
B2G-EDS, Hostaperm Yellow H4G-EDS, Novoperm Yellow HR-70-EDS,
Hostaperm Green GNX, BASF Irgazine red L 3630, Cinquasia Red L 4100
HD, and Irgazin Red L 3660 HD; Sun Chemical phthalocyanine blue,
phthalocyanine green, diarylide yellow or diarylide AAOT yellow.
Color particles can also be formed from inorganic pigments, such as
CI pigment blue 28, CI pigment green 50, CI pigment yellow 227, and
the like.
[0019] The surface of the charged particles may be modified by
known techniques based on the charge polarity and charge level of
the particles required, as described in U.S. Pat. Nos. 6,822,782
and 7,002,728, US Publication Nos. 2014-0011913, US 2012-0199798,
and US 2013-0175479, the contents of all of which are incorporated
herein by reference in their entirety.
[0020] The charged particles may exhibit a native charge, or may be
charged explicitly using a charge control agent, or may acquire a
charge when suspended in a solvent or solvent mixture.
[0021] Suitable charge control agents are well known in the art;
they may be polymeric or non-polymeric in nature or may be ionic or
non-ionic. Examples of charge control agent may include, but are
not limited to, Solsperse 17000 (active polymeric dispersant),
Solsperse 9000 (active polymeric dispersant), OLOA 11000
(succinimide ashless dispersant), Unithox 750 (ethoxylates), Span
85 (sorbitan trioleate), Petronate L (sodium sulfonate), Alcolec
LV30 (soy lecithin), Petrostep B100 (petroleum sulfonate) or B70
(barium sulfonate), Aerosol OT, polyisobutylene derivatives or
poly(ethylene co-butylene) derivatives, and the like.
[0022] The solvent in which the pigment particles are dispersed has
a dielectric constant in the range of about 2 to about 30,
preferably about 2 to about 15 for high particle mobility. Examples
of suitable dielectric solvent include hydrocarbons such as isopar,
decahydronaphthalene (DECALIN), 5-ethylidene-2-norbornene, fatty
oils, paraffin oil; silicon fluids; aromatic hydrocarbons such as
toluene, xylene, phenylxylylethane, dodecylbenzene and
alkylnaphthalene; halogenated solvents such as perfluorodecalin,
perfluorotoluene, perfluoroxylene, dichlorobenzotrifluoride,
3,4,5-trichlorobenzotri fluoride, chloropentafluoro-benzene,
dichlorononane, pentachlorobenzene; and perfluorinated solvents
such as FC-43, FC-70 and FC-5060 from 3M Company, St. Paul Minn.,
low molecular weight halogen containing polymers such as
poly(perfluoropropylene oxide) from TCI America, Portland, Oreg.,
poly(chlorotrifluoro-ethylene) such as Halocarbon Oils from
Halocarbon Product Corp., River Edge, N.J., perfluoropolyalkylether
such as Galden from Ausimont or Krytox Oils and Greases K-Fluid
Series from DuPont, Del., polydimethylsiloxane based silicone oil
from Dow-corning (DC-200).
[0023] The color of a solvent may be generated by dissolving a dye
in the solvent. Solvent dyes are generally commercially available.
The molecules of dyes for organic solvents are typically non-polar
or of a low polarity, and they do not undergo ionization. The
preferred dyes have good light fastness (e.g., metal complex
based). Examples of commercially available solvent dyes may
include, but are not limited to, Solvent Red 24, Solvent Red 26,
Solvent Red 164, Solvent Yellow 124 or Solvent Blue 35.
[0024] One of the unique features of the display of the present
invention is that, unlike traditional electrophoretic displays, the
charged particles in the present invention exhibit no or minimum
bistability, which means that when a driving voltage is turned off,
the particles would move away from the positions where they have
been driven to by a driving voltage.
[0025] FIG. 2 illustrates how different color states are displayed
with the display of FIG. 1. It is assumed, for illustration
purpose, both the positively charged and negatively charged
particles are of the white color and they are dispersed in a
solvent of a blue color.
[0026] When a positive driving voltage is applied to a pixel (see
2(a)), the positively charged white pigment particles are driven to
be at or near the common electrode. As a result, a white color is
seen at the viewing side.
[0027] When a negative driving voltage is applied to a pixel (see
2(b)), the negatively charged white pigment particles are driven to
be at or near the common electrode. As a result, the color seen at
the viewing side is also the white color.
[0028] When the driving voltage applied to the pixel of 2(a) or
2(b) is turned off, the charged white particles, due to lack of
bistability, would move away from the common electrode. In this
case, the color of the solvent (i.e., blue) is seen at the viewing
side (see 2(c)).
[0029] From a pixel standpoint, a pixel displays the same optical
state regardless of whether a positive or negative driving voltage
is applied, and a different optical state is displayed when the
driving voltage is turned off.
[0030] More specifically, when there is an electric field generated
by a positive driving voltage, which is sufficient to drive the
positively charged particles to reach the common electrode side to
allow the color of the particles to be seen at the viewing side,
such a state is referred to as an "on" state. Likewise, when there
is an electric field generated by a negative driving voltage, which
is sufficient to drive the negatively charged particles to reach
the common electrode side to allow the color of the particles to be
seen at the viewing side, this state is also referred to as the "on
state". As stated above, because the two types of particles are of
the same color, the colors displayed at both "on" states are the
same.
[0031] When the power is off (that is, no driving voltage is
applied), the state is referred to as an "off" state. At the "off"
state, a different color is displayed (that is, seen at the viewing
side).
[0032] According to the present invention, due to the lack of
bistability, the color change between the "on" and "off" states,
defined as .DELTA.E, is at least 5, preferably at least 10.
[0033] In color science, L, a, and b are used to define an optical
state. A Lab color space is a color-opponent space with dimension L
for lightness, and a and b for the color-opponent dimensions, based
on nonlinearly compressed (e.g. CIE XYZ color space) coordinates.
The lightness, L*, represents the darkest black at L*=0, and the
brightest white at L*=100. The color channels, a* and b*, will
represent true neutral gray values at a*=0 and b*=0. The red/green
opponent colors are represented along the a* axis, with green at
negative a* values and red at positive a* values. The yellow/blue
opponent colors are represented along the b* axis, with blue at
negative b* values and yellow at positive b* values. The scaling
and limits of the b* and b* axes will depend on the specific
implementation of Lab color, but they often run in the range of
.+-.100 or -128 to +127. Every perceivable color has a set of L*,
a* and b* values.
[0034] If two color states are expressed as (L.sub.1*, a.sub.1*,
b.sub.1*) and (L.sub.2*, a.sub.2*, b.sub.2*), respectively, then
the color difference between the two color states, .DELTA.E, can be
obtained from the following equation:
.DELTA.E= {square root over
((L.sub.2*-L.sub.1*).sup.2+(a.sub.2*-a.sub.1*).sup.2+(b.sub.2*-b.sub.1*).-
sup.2)}{square root over
((L.sub.2*-L.sub.1*).sup.2+(a.sub.2*-a.sub.1*).sup.2+(b.sub.2*-b.sub.1*).-
sup.2)}{square root over
((L.sub.2*-L.sub.1*).sup.2+(a.sub.2*-a.sub.1*).sup.2+(b.sub.2*-b.sub.1*).-
sup.2)}
[0035] For example, if at an "on" state, a pixel displays a white
color, expressed in the Lab system as (65, -3.3, -1.8) and at the
"off" state, the pixel displays a blue color, expressed as (32.6,
-11.4, -34.5). In this example, the .DELTA.E is calculated to be
46.7.
[0036] FIG. 3 illustrates a second aspect of the present invention.
In this figure, an electrophoretic fluid comprises two types (31
and 32) of charged particles of the same color, but carrying
opposite charge polarities. The two types of charged particles are
dispersed in a solvent or solvent mixture (33) which may be clear
and colorless. The display fluid is sandwiched between two
electrode layers. One of the electrode layers is a common electrode
(35) which is a transparent electrode layer (e.g., ITO), spreading
over the entire top of the display device. The other electrode
layer (36) is a layer of pixel electrodes (36a). The space between
two dotted vertical lines represents a pixel. Therefore each pixel
corresponds to a pixel electrode.
[0037] The descriptions above of the first aspect of the present
invention are applicable to this aspect of the invention. However,
in this aspect of the invention, a third type of particles (34) is
added and the third type of particles is uncharged or slightly
charged, and of a color which is visually in contrast with the
color of the two types of charged particles. The solvent (33) in
which the particles are dispersed preferably is clear and
colorless.
[0038] The uncharged or slightly charged color particles may have a
zeta potential of <20 mV. Therefore they will remain stationary
and substantially uniformly dispersed in the display fluid, during
operation of the display device.
[0039] The uncharged or slightly charged particles may be formed
from a polymeric material. The polymeric material may be a
copolymer or a homopolymer.
[0040] Examples of the polymeric material for the uncharged or
slightly charged particles may include, but are not limited to,
polyacrylate, polymethacrylate, polystyrene, polyaniline,
polypyrrole, polyphenol, polysiloxane or the like. More specific
examples of the polymeric material may include, but are not limited
to, poly(pentabromophenyl methacrylate), poly(2-vinylnapthalene),
poly(naphthyl methacrylate), poly(alpha-methystyrene),
poly(N-benzyl methacrylamide) or poly(benzyl methacrylate).
[0041] In one embodiment, if the uncharged or slightly charged
particles are colored, they may include, but are not limited to,
commercially available colorants used in the LCD industry for color
filter applications, such as Clariant's Hostaperm Red D2B-COF VP
3781 (i.e., red 254) which is in the class of diketopyrrolopyrrole,
Hostaperm Blue E3R-COF VP3573 (i.e., blue 15:6) which is in the
class of phthalocyanine, or Hostaperm Violet RL-COF O2 VP3101
(i.e., violet 23) which is in the class of dioxazine.
[0042] In another embodiment, the uncharged or slightly charged
colored particles may have a transparent polymeric matrix and with
dye molecules solubilized in the matrix. Examples of this type of
uncharged or slightly charged colored particles may include, but
are not limited to, dyed polymeric microparticles and dyed
polystyrene particles all of which are commercially available.
[0043] The size of the uncharged or slightly charged particles is
preferably in the range of 10 nanometers to 5 microns, more
preferably 50 nanometers to 2 microns.
[0044] The concentration of the charged particles in FIG. 3 may be
1% to 20% by volume, preferably 5% to 10% by volume, in the display
fluid and the concentration of the uncharged or slightly charged
particles may be 1% to 30% by volume, preferably 7% to 20%, in the
display fluid.
[0045] FIG. 4 illustrates how different color states are displayed
with the display fluid of FIG. 3. It is assumed, for illustration
purpose, both the positively charged and negatively charged
particles are of the white color and the third type of pigment
particles are uncharged or slightly charged and of a blue color.
The solvent, in this example, is clear and colorless.
[0046] When a positive driving voltage is applied to a pixel (see
4(a)), the positively charged white pigment particles are driven to
be at or near the common electrode. As a result, a white color is
seen at the viewing side.
[0047] When a negative driving voltage is applied to a pixel (see
4(b)), the negatively charged white pigment particles are driven to
be at or near the common electrode. As a result, the color seen is
also the white color.
[0048] When the driving voltage applied to the pixel of 4(a) or
4(b) is turned off, the charged white particles, due to lack of
bistability, would move away from the common electrode. In this
case, the color (i.e., blue) of the uncharged or slightly charged
particles is seen at the viewing side (see 4(c)).
[0049] As shown in FIG. 4, a pixel displays the same optical state
regardless of whether a positive or negative driving voltage is
applied, and a different optical state is displayed when the
driving voltage is turned off. According to the present invention,
the color change between the "on" and "off" states, defined as
.DELTA.E, is also at least 5, preferably at least 10.
[0050] There are many different ways to reduce/eliminate
bistability of charged particles in order to achieve the .DELTA.E
being at least 5, preferably being at least 10. The following are a
few options.
[0051] For example, the higher .DELTA.E may be achieved by surface
modification of the charged particles. In one embodiment, the
particles may be coated with a solvent-soluble polymer to cause the
particles to be compatible with the solvent in which they are
dispersed.
[0052] In general, the presence of the solvent-soluble polymer on
the particle surface is key to achieve good dispersability of the
particles. Selection of the solvent-soluble polymer would depend on
the compatibility of the material with the solvent used in an
electrophoretic fluid. Suitable polymers may include, but are not
limited to, polyethylene, polypropylene, polyacrylate, or
polysiloxane.
[0053] The solvent-soluble polymers may be formed from the
monomers, oligomers or polymers, and they may have a single chain
or a branched chain. They may also have different configurations,
such as coils, stretched chains or irregular tangled chains on the
particle surface, depending on compatibility of the polymer with
the solvent in which the particles are dispersed and/or the density
and length of the polymer chains.
[0054] On the surface of the particles, there may be only one
single type of the solvent-soluble polymer or several different
types of solvent-soluble polymer.
[0055] One example is TiO.sub.2 as core pigment, surface coated
with polylauryl acrylate. Other suitable monomers forming the
solvent-soluble polymers may include, but are not limited to,
lauryl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl
methacrylate, hexyl acrylate, hexyl methacrylate, n-octyl acrylate,
n-octyl methacrylate, n-octadecyl acrylate or n-octadecyl
methacrylate.
[0056] If the charged particles are surface coated with a
solvent-soluble polymer, and when a driving voltage is turned off,
the particles would tend to move away from the locations where they
were driven to, and be dispersed back into the solvent.
[0057] Alternatively, the size of the charged particles is
preferably relatively large, for example, larger than 1 micron. The
density of the charged particles is also preferably high, for
example, at least twice as high as the density of the solvent in
which the particles are dispersed.
[0058] The fluid may also be designed to have a low shear viscosity
in the range of 0.5 to 50 cps, at room temperature, so that the
charged particles may be easily settle downwards due to gravity or
uniformly dispersed by Brownian movement, when the power is
off.
[0059] Further alternatively, a dielectric coating may be applied
to the surface of the common electrode (which is on the viewing
side) to cause the common electrode to have a remnant voltage
(similar to the reverse bias phenomenon) which can push back the
particles from the common electrode when the power is off. The
dielectric coating may be polyurethane, polyacrylate,
polyvinyalcohol, polyvinylacetate, epoxy or the like.
[0060] The electrophoretic fluid of the present invention is filled
in display cells. The display cells may be cup-like microcells as
described in U.S. Pat. No. 6,930,818, the content of which is
incorporated herein by reference in its entirety. The display cells
may also be other types of micro-containers, such as microcapsules,
micro-channels or equivalents, regardless of their shapes or sizes.
All of these are within the scope of the present application.
[0061] While the present invention has been described with
reference to the specific embodiments thereof, it should be
understood by those skilled in the art that various changes may be
made and equivalents may be substituted without departing from the
true spirit and scope of the invention. In addition, many
modifications may be made to adapt a particular situation,
materials, compositions, processes, process step or steps, to the
objective and scope of the present invention. All such
modifications are intended to be within the scope of the claims
appended hereto.
* * * * *